Many patents disclose magnetic encoders, in many cases related to automobile ABS systems, In all of these cases the sensors must be protected from the environment. Generally the sensors are separated from the wheel by a sheet of a plastics material.
U.S. Pat. No. 7,229,708 (Abe) issued Jun. 12, 2007 discloses a magnetic encoder using a stainless steel sheet and a resin laminated layer to provide water resistance.